(1) Field of the Invention
The present invention relates to a single-chip microcomputer, and more particularly, to a single-chip microcomputer having means for inhibiting read-out of the contents of an internal ROM.
(2) Description of the Prior Art
Generally, in order to expand the available address space, a single-chip microcomputer provided with an internal read-only memory (ROM) can be connected through an interface to an external ROM. The external ROM usually has a first region in which the address space is different from that of the internal ROM for storing programs, and a second region in which the address space is the same as that of the internal ROM. The single-chip microcomputer, when connected to the external ROM, can be switched between a first mode of operation and a second mode of operation, in response to a switching signal, for example, an external access signal used when an external device is accessed. In the first mode of operation, the internal ROM and the second region of the external ROM are accessable. In the second mode of operation, only the external ROM can be accessed unless a specific program such as later described in detail is stored in the external ROM.
In the single-chip microcomputer, when connected to the external ROM, it is desired, in the interests of secrecy, that the contents of the above-mentioned internal ROM cannot be read out to an external device.
Conventionally, countermeasures have been taken to make it impossible to transfer the contents of the internal ROM to an external device by, for example, cutting a fuse between the internal ROM and the external portion, after the contents of the ROM are determined, in the manufacturing process, to be normal.
However, when the second region of the external ROM is accessed in the second mode of operation, the contents of the internal ROM can be easily read out to an external device by switching the external access signal so as to select the first mode of operation, when the second region of the external ROM stores a program for reading out the contents of the internal ROM.